Liquid crystal and other flat panel displays are increasingly being utilized for various applications, from color televisions and computer monitors to roadside signs. Such devices are typically fabricated utilizing well-known clean room fabrication techniques. Nonetheless, dust, other micro-particles and other factors often lead to defects, such as open circuits or short circuits, in the increasingly high-density environment of an active matrix liquid crystal display. The poor manufacturing yields, which may be as low as ten percent (10%), as well as the time and expense required to test such devices, have greatly contributed to the fabrication costs associated with such devices.
Initially, flat panel displays were not tested until the final assembly stage, when the addressing circuitry, referred to as the active plate, was integrated with the liquid crystal material. The testing process consisted of activating the final display, and visually inspecting the device for defects. At this final stage, however, identified defects could not be repaired in a practical manner.
It was observed that most defects occurred in the active plate. Thus, in order to increase the manufacturing yield associated with flat panel displays, in process testing of the active plates, before the liquid crystal material was applied, was proposed. Initially, the in-process testing methods consisted of a probing method to electrically access each individual row and column intersection of the pixel array. Although such contact techniques effectively identified defects in an active plate, prior to final assembly, they were extremely time-consuming.
In addition, in-process voltage imaging techniques, such as those developed by Photo Dynamics, Inc., and described, for example, in U.S. Pat. No. 5,570,011 to Henley, entitled "Method For Testing a Device Using Voltage Imaging," utilize an electro-optical probe to image the electric field at a pixel. Again, although such voltage imaging techniques effectively identify defects in an active plate, prior to final assembly, the electro-optical probe has a narrow field of view and must be positioned a short distance from the plate. Thus, a time-consuming step-and-repeat approach must be implemented to image an entire active plate.
It is known to test printed circuit boards (PCBs) and other circuits fabricated with VLSI techniques using infrared (IR) thermography. Generally, IR thermography techniques detect the IR emission of the circuit being tested when an electric signal is applied and compare the heating response characteristics to a known defect-free sample. U.S. Pat. No. 3,991,302 to Danner, entitled "Method For Detecting and Isolating Faults in Digital and Analog Circuits With Multiple Infrared Scanning Under Conditions of Different Stimuli," for example, discloses a method for detecting faults in circuits based on infrared heating response characteristics.
As apparent from the above-described deficiencies with conventional systems for testing an active plate of a flat panel display, a need exists for an improved non-invasive diagnostic method for in-process testing. A further need exists for a faster and more efficient diagnostic method for in-process testing. Yet another need exists for a diagnostic method for in-process testing using commercially available components. Another need exists for a diagnostic method for in-process testing that permits defects to be localized to facilitate repair and tolerates imaging at further distances from the active plate.